Prostaglandin synthetase is capable of metabolizing a number of substances previously thought to be metabolized only by the mixed-function oxidase system. A method has been developed for testing the mutagenicity of polycyclic hydrocarbons in the presence of prostaglandin synthetase. Benzo(a)pyrene, benzanthracene and their non-mutagenic metabolites will be tested for mutagenicity using this system after activation by prostaglandin synthetase.